The present invention relates to a receiver that receives and detects a wireless signal.
An electronic key system known in the art automatically locks or unlocks the doors of a vehicle through wireless communication performed between an electronic key, which is carried by a driver, and the vehicle. A vehicle provided with such an electronic key system normally includes a transmitter, which transmits various wireless signals to the electronic key, and a receiver, which receiving various wireless signals transmitted from the electronic key. Japanese Laid-Open Patent Publication No. 11-215040 describes a receiver that uses a diversity scheme to improve the signal reception condition.
With the diversity scheme, among a plurality of antennas, the one having the best signal reception condition is used to perform communication. As shown in FIG. 1, a receiver employing the diversity scheme includes, for example, two antennas 101 and 102, a distributor 110, a reception circuit 109, an envelope wave detector 105, a signal level detection unit 104, and a control unit 106. The antennas 101 and 102 each output a received signal to the distributor 110. The distributor 110 distributes the received signal to the envelope wave detector 105 and the reception circuit 109. The reception circuit 109 demodulates the received signals from the antennas 101 and 102 to generate a data signal and outputs the data signal to various devices. The envelope wave detector 105 detects the envelope of the signal from each of the antennas 101 and 102. In the envelope detection, the envelope wave detector 105 retrieves only the envelope of the signal. The signal detected by the envelope wave detector 105 is provided to the signal level detection unit 104. The signal level detection unit 104 detects the signal level of the detected signal and provides the detection result to the control unit 106.
The receiver further includes a switching element 108, which selectively connects one of the antennas 101 and 102 to the distributor 110. The control unit 106 switches the antennas 101 and 102 by controlling the switching element 108 during reception of wireless signals. The control unit 106 compares the signal levels of the antennas 101 and 102 detected by the signal level detection unit 104 to determine which one of the antennas 101 and 102 is in a better condition for signal reception. This determination enables the receiver to communicate under a better signal reception condition by using the antenna having the better signal reception condition.
The receiver arranged in the vehicle may perform wireless communication with a device other than the electronic key. An example of such a device is a tire pressure monitoring system (TPMS), which monitors the air pressure of tires of a vehicle. A sensor unit is arranged in each tire of the vehicle including the TPMS to detect the air pressure of the tire. Each sensor unit transmits an air pressure signal including information on the detected air pressure of the tire to the receiver. The amount of information included in the air pressure signal is smaller than the amount of information included in a signal transmitted to lock or unlock the doors. The amount of information of the signal for locking or unlocking the doors is larger because such a signal requires high security. The signal length of the air pressure signal is short. Thus, the transmission of the air pressure signal may be completed within a short period of time. In this case, the transmission of the air pressure signal may be completed when the control unit 106 is still comparing the signal levels of the different antennas detected by the signal level detection unit 104. Also, even if the signal reception condition of the antennas 101 and 102 were to be determined before completion of the transmission of the air pressure signal, only part of the air pressure signal may be received by the antenna having the better signal reception condition. In particular, when one antenna 101 is able to receive a signal and the other antenna 102 is not able to receive a signal, a delay in the switching of the antennas may hinder normal reception of the signal.
As described above, a receiver employing the diversity scheme may not be able to optimize the signal reception condition due to the execution time required for the process of determining the signal reception condition by comparing the signal levels. This problem is not limited to a receiver for a vehicle and may also occur when performing communication with a mobile unit that employs the diversity scheme. The above problem also occurs with a variable directivity antenna, which can switch the antenna directivity, when determining the directivity that obtains an optimum signal reception condition.